1. Field of the Invention
The present invention relates generally to the transmission of information between a downhole location and a surface location, and more specifically to an apparatus and method for the transmission of information between downhole and surface locations during the conduct of a subterranean drilling operation using air or gas as the energy source for a downhole drilling motor.
2. State of the Art
Drilling for oil and gas with downhole motors employing dry air, mist, or foams (all referred to hereinafter generically as "air") as a drilling fluid has been contemplated and practiced with some limited success for a number of years. Use of air as the drilling fluid, because of its low density, can result in faster penetration rates. Moreover, air drilling is less damaging to the producing formation than oil- or water-based drilling fluids. However, the reduced hydrostatic head of the air drilling fluid cannot effectively control formation pressures nor support borehole wall against collapse, and therefore air drilling is substantially limited to competent formations and requires religious use of blow-out preventors ("BOP's").
The foregoing limitations notwithstanding, air drilling has many applications, and improved motor technology has popularized its use in recent years, particularly in navigational drilling operations where a bottomhole assembly including a drilling motor may be steered to drill either a curved path or straight ahead. When drilling a nonlinear path, the bottomhole assembly is oriented in a particular direction, and drilling proceeds under power of the motor alone. For straight ahead drilling, the drillstring is rotated to negate the drill bit tilt angle or offset from the longitudinal axis of the bottomhole assembly. One suitable and recently developed bottom hole assembly for air drilling is the Navi-Drill Mach 1/AD, employed by Eastman Christensen Company of Houston, Tex., which assembly includes a positive displacement Moineau-type air motor and an adjustable bent sub between the motor and the drill bit, the bent sub providing the desired bit tilt angle for nonlinear drilling. An additional bent sub may be placed above the motor to enhance the assembly's kick off abilities, but such an arrangement precludes drillstring rotation and straight ahead drilling.
When drilling directionally or navigationally it is, of course, imperative to track the azimuth and inclination of the actual borehole against the intended well plan. Many survey, steering and measurement-while-drilling ("MWD") devices and techniques have been developed and employed over the years, but experience has confirmed many deficiencies and limitations of the prior art apparatus and methods when employed in an air drilling environment.
Conventional survey instrumentation, and particularly high accuracy gyroscopic instrumentation, is somewhat delicate for use in air drilling, as the drilling fluid does not provide dampening of deleterious vibration and resonance effects. Moreover, when conducting a navigational drilling operation, drilling torque may drastically change the toolface orientation and thus the borehole path over a short drilling interval, and survey techniques only confirm such changes after the fact.
Conventional MWD systems employ pressure pulses in the drilling fluid to transmit information from the downhole probe to the surface. As air is highly compressible, it cannot be pulsed effectively, and so conventional mud-pulse MWD technology is inoperative in air-drilled boreholes. Electromagnetic MWD ("EM MWD") systems, which employ the drillstring as the transmission media for electromagnetic waves, have been employed in air-drilled holes with mixed results. Rougher drilling conditions in air-drilled holes commonly cause tool failure, and EM MWD use can be severely hampered by formation resistivity. Finally, use of EM MWD requires a conductive drilling fluid, and therefore cannot be used for dry air drilling.
A steering tool offers significant advantages while navigationally drilling, as it provides continual surface readout of survey data while drilling, including the highly important toolface readout, solving the problem of reactive torque effects causing toolface orientation change. Steering tools also offer almost instantaneous information, unlike MWD tools, which do not continuously transmit data between the downhole location and the surface. Wireline-controlled steering systems have been employed in directional drilling, such systems including a side-entry sub and split kelly for the wireline to maintain contact with the probe. With a side-entry sub, the wireline is on the outside of the drillstring, and therefore subject to kinking, wear and breakage. If the probe signal is lost, the drillstring must be pulled out of the hole to the location of the side-entry sub, and the probe retrieved. Moreover, these systems preclude rotation of the drillstring due to the exterior location of the wireline. If a swivel assembly is used instead of a side-entry sub, the steering tool must be round-tripped out of the hole whenever a drill pipe joint connection is made, although in this case the drillstring may be rotated for straight ahead drilling. Finally, use of a wireline exterior to the drillstring precludes full closure of the BOP's unless the wireline is seuered.
Wet-connect systems have been developed wherein a steering tool probe having a wireline leading to a connection on the upper end thereof is run into the drillstring at the kickoff point, the upper end clamped off at the connection, and an upper wireline section with a mating connection on the lower end thereof is run into the drillstring to electrically connect the probe for directional drilling. While effective, such systems cause lost rig time due to the necessity for wireline retrieval prior to drillstring rotation.
Horizontal air-drilled wells provide additional problems as, at well inclinations exceeding 70 degrees from the vertical, a steering or survey tool will no longer fall down the drillstring, nor will air passing by the tool generate enough drag to carry it downhole. Currently, two methods are used to address this problem. In the first, the drillstring is pulled from the hole until the bit is at 70 degrees of inclination, a side-entry sub installed and a survey or steering tool run on electric line to a latching assembly above the drill bit, and the drillstring tripped back to bottom with the wireline above the side entry sub on the outside of the drillstring. A survey is then taken, the drillstring tripped back out to the side-entry sub, the survey tool and side-entry sub removed, and the drillstring run back to bottom to continue drilling. Obviously, a great deal of rig time is wasted with this method, and the driller learns of deviations from the well plan after the fact. The second method reduces time somewhat, by running a survey tool on a slickline with a releasing overshot when the drillstring has been pulled to the 70 degree inclination point. Upon reaching the monel drill collars, a monel sensor activates the releasing overshot, disconnecting the survey tool from the slick line, which is then removed from the hole. The drillstring is tripped back to the bottom to take the survey, subsequent to which the drillstring is pulled to 70 degrees, and the survey tool retrieved with a standard overshot run in on slickline. It will be appreciated that significant rig time is still involved with this method.